1. Field of the Invention
The present invention relates to a cathode assembly used in an electron gun of a picture tube, and more particularly, is an invention for preventing the spread of metal vapor produced from near an emitter to a hole in a cylindrical insulator.
2. Description of the Related Art
The picture tube used in a television receiver is called a Braun tube or a CRT (cathode ray tube). An electron gun and a phosphor screen are sealed in a funnel shaped glass tube (glass envelope).
The cathode assembly constituting the electron gun has the cathode inserted in the through-hole of the cylindrical insulator such as ceramic, in a state supported by the sleeve holder. The emitter attached to the front end of the cathode is affixed, at a predetermined distance relative to the cylindrical electrode of the first grid provided on the cylindrical insulator such as ceramic. Further, by heating and activating the emitter, hot electrons are emitted. The electron beam is accelerated and controlled to converge by a plurality of cylindrical electrodes, including the first grid and the second grid.
When the cathode assembly is operated and made to emit hot electrons, the oxide constituting the emitter is decomposed and gas is produced. While the glass envelope where the cathode assembly is provided is maintained at a high vacuum state, the degree of vacuum inside the through-hole of the cylindrical insulator locally falls due to the gas produced at the time of the activation of the oxide, so holes are opened in the cylindrical insulator. Due to the provision of the holes, the gas produced in the through-hole can be made to disperse inside the glass envelope and the hot electrons can be suitably emitted.
However, since the cathode is operated at a high temperature of close to 1000.degree. C., the metal cylinder (hereinafter referred to as the "sleeve") constituting the cathode is heated and therefore metal vapor is produced. Further, this metal vapor spreads to the outside of the cathode assembly from the holes opened in the cylindrical insulator. It deposits on the cylindrical insulator of the other adjoining cathode assembly.
In this way, if the metal vapor spread from the holes deposits on the outer surface of an insulator such as ceramic, there is a danger of conduction between the sleeve holder supporting the cathode and the cylindrical electrode of the first grid etc. or the spacer provided between the sleeve holder and the cylindrical electrode and therefore the function of the electron gun is impaired.
It is true that if the holes are not made in the long side of the cylindrical insulator, but made in the short side, the spreading of the metal vapor from the holes will not become a cause of leakage, but with this structure, it is not possible to ensure a sufficient area of the opening of the holes and further the strength of the cylindrical insulator is lowered, so this is not practical.